Siderite-bearing apatite nodules occur in the Miocene Aoki Formation in the Ueda area of Nagano Prefecture. The nodules are classified into two types petrographically: one is nodules in which authigenic mineral is pyrite and apatite and another is those consisting of apatite and siderite. The nodules from the study area do not carry calcite. In order to clarify the diagenetic stage of apatite precipitation and the reason for the absence of calcite in the nodules, TCC (total carbonate content) , the weight ratio of siderite to apatite in the siderite-apatite nodules, the presence or absence of SO₄^2-in apatite, the carbon and oxygen isotopic composition of siderite, were measured.

It is clear that the apatite precipitated mainly in the sulphate reduction stage of early diagenesis, since nearly half of the apatite samples examined contain SO₄^2-in its crystal lattice. The δ¹³C of siderite ranges from 1.9 to 9.0 ‰, and δ¹³C decreases with the decrease in δ¹⁸O. The Isotopic data indicate that siderite precipitated at the diagenetic zones ranging from methane fermentation to organic matter pyrolysis. It is concluded that the precipitation of apatite is preceded by that of siderite. This leads to a following consequence: The porosity of sediment at the period of apatite precipitation is represented by the TCC of apatite nodules and mixed siderite apatite nodules, and that of siderite precipitation is by the TCC of siderite nodules and the volume percentage of siderite in mixed siderite apatite nodules. There are some samples in which the TCC of apatite nodules falls within the porosity range of siderite precipitation. From this, it was clarified that the precipitation of apatite was not restricted to the sulfate reduction zone, but continued to the deeper zones. In the Aoki sediment, the pH of pore water fell below that of seawater in the early stage of diagenesis. Consequently, calcite did not crystallize from pore water, forming calcite-absent nodules.

The Early Cretaceous Yotsumatayama quartz diorite mass is a member of the Atokura Nappe in the Shimonita area, and consists mostly of quartz diorite with small amounts of granodiorite. The quartz diorite is composed mainly of hornblende, plagioclase and minor quartz, and while the granodiorite is composed of biotite, muscovite, plagioclase, quartz, K-feldspar with minor amounts of garnet and cordierite. The granodiorite includes metamorphic xenoliths such as biotite gneiss, amphibolite and crystalline limestone. Biotite gneiss is characterized by layered texture, composed of melanocratic and leucocratic layers. Granodiorite is heterogeneous in appearance around the biotite gneiss xenoliths, where magmatic fabric of biotite-enriched schlieren is developed. Granodiorite is closely associated with foliated granodiorite, leucocratic granodiorite and pegmatitic granodiorite. We observed in outcrops that the biotite gneiss xenoliths gradually grade into granodiorite. This observation suggests that the biotite gneiss xenoliths are assimilated into the granodioritic magma. The chemical composition of granodioritic magma surrounding the biotite gneiss xenoliths changes to peraluminous variety due to the assimilation of the xenoliths in sedimentary origin．Therefore, these granodiorites around the biotite gneiss xenoliths are considered to show the mineral assemblages of the S-type granitoid.